The present invention relates to systems and methods for inspecting objects, particularly cargo in containers, trucks and trains, using spectrally filtered penetrating radiation.
X-ray inspection of containers is well established for many purposes including the search for contraband, stolen property and the verification of the contents of shipments that cross national borders.
Such inspection systems typically produce a so-called transmission image by measuring the intensity distribution of the x-rays that traverse the container. The contents of lightly loaded containers can be effectively examined with x-ray energies in the hundreds of keV range but heavily loaded semi-trailers, tanker trucks, trains, and ISO containers typically require beams of x-rays in the MeV range. The design of an x-ray inspection system to examine heterogeneous cargo requires joint consideration of conflicting requirements for penetration, sensitivity and radiation dosage, for which the following first-order criteria give general guidance.
Penetration: As the energy of the x-rays increases, with the electron beam power kept constant, the penetration increases inversely with the linear absorption coefficient, xcex cmxe2x88x921. For example, the high-energy x-ray components of the beam from a 3 MeV accelerator penetrate approximately 3 times farther through iron than do the high-energy components from a 450 keV x-ray generator; the relevant linear absorption coefficients in iron decrease by almost that factor of 3.
Sensitivity: The sensitivity of the x-rays to a given thickness xcex94x of an object depends on the material surrounding the object as well as the material of the object itself. Other parameters being equal, the sensitivity S is usefully defined as the fractional change in the count rate I per centimeter of the material. That is,       S    =                            Δ          ⁢                      xe2x80x83                    ⁢                      I            /            I                                    Δ          ⁢                      xe2x80x83                    ⁢          x                    =              -        λ              ,
where xcex94I is the decrement in count rate due to material of thickness xcex94x. The sensitivity is greatest at the lowest energy (highest value of xcex) of penetrating radiation. In the above example, the sensitivity of the high energy x-rays has fallen by a factor of almost 3 on increasing the energy from 450 keV to 3 MeV, and it takes almost 9 times as many x-rays to obtain the same statistical accuracy for the measured attenuation.
The conflict between penetration and sensitivity is generally met by accepting the full spectrum of x-rays generated by the electron beam. The high energy components are effective for maximum penetration while the far more copious lower energy components have the sensitivity needed to most effectively examine lightly loaded containers or containers with lightly loaded sections.
Radiation dose: The radiation dose in a beam of x-rays generated by an electron of energy E striking a tungsten target is approximately proportional to E3, as discussed by Buechner, et al., Physical Review, vol. 74, pp. 1348ff, (1948). At constant power, the direct radiation dose in the beam varies approximately as E2, as does the ambient radiation dose. As used in this description and in any appended claims, the term xe2x80x9cambient radiation dosexe2x80x9d refers to the dose in the surroundings from scattered radiation.
In the example above, with the electron power kept constant, the ambient radiation dose rises by almost a factor of 40 as the electron energy is raised from 450 keV to 3 MeV. In order to satisfy the radiation safety codes it becomes necessary to add costly shielding that generally involve strengthened infrastructure and massive doors to the inspection area. In the United States, the inspection system must keep the dose to areas accessible to people to less than 5 xcexcSv/hr. (21 CFR xc2xa71020.40) These specifications are referred to as xe2x80x9ccabinetxe2x80x9d specifications. The phrase xe2x80x9copen cabinetxe2x80x9d as used herein refers to an enclosure, with or without doors or roof, that meets legal cabinet regulation specifications without the need for radiation shields other than in the vicinity of the beam.
To meet the demands of inspecting densely-loaded cargo, some current systems apply x-rays of energy higher than 450 keV. It has been assumed and taught in the art that higher energy results in high ambient radiation that requires extensive shielding and greatly increased costs to comply with regulations.
Some currently employed cargo inspection systems use pulsed linear accelerators (linacs) operating in the range from about 3 MeV to 9 MeV, producing fan beams of x-rays that pass through the cargo into a linear array of x-ray detectors that measure their intensity. The copious low energy components of the fan beam produce high quality images of lightly loaded containers while the high-energy components have sufficient penetrating power to find contraband behind more than 30 cm of steel. The radiation produced by the linacs is very high, typically greater than 1 Gray/min at a meter, and so too is the cost for the radiation enclosure and shielding required. The cost of the building, with radiation-containing doors for the inspection tunnel often exceeds the cost of the x-ray inspection system itself.
Current uses of spectrally selective absorbers in the context of x-ray beam generation include their use in MeV x-ray generators to flatten the intensity distribution as a function of angle. Absorbers are also used in medical applications to minimize the radiation dose to the patient by eliminating the low-energy components that are absorbed preferentially in the skin and near-surface tissues and play no useful role in the diagnosis or treatment. Finally, absorbers are used to create quasi-monochromatic beams of x-rays, as for dual-energy analysis, for example. Ambient radiation, as defined above, figures in none of these instances of current use of absorbers, neither as a design consideration nor to provide active control of the x-ray intensities.
In accordance with systems and methods described herein, both the high energy and low-energy components of MeV x-ray generators may be used while keeping the ambient radiation low enough so that the cost of the infrastructure to contain the radiation is significantly reduced. Thus, the seemingly mutually exclusive needs for penetration and sensitivity to easily penetrable objects may advantageously be obtained. This may be accomplished, in accordance with the present invention, by combining several modalities: First, a highly penetrating, high-energy x-ray beam with minimal ambient radiation can be obtained by properly shaping the spectrum of the x-ray beam from the accelerator by means of an appropriate absorber. Second, the essential lower-energy x-ray spectrum, generated by the same or a different accelerator, can satisfy the needs for sensitivity to easily penetrable objects while maintaining minimal ambient radiation. Third, the two spectra can be coordinated in time so that there is no interference or cross-talk between the two components. Additional embodiments of the invention also make use of the fact that the intensities of the x-ray generators can be controlled to maximize the effectiveness of the inspection system while maintaining xe2x80x9copen cabinetxe2x80x9d status. One method of control uses the low energy transmission system to control the intensity of the beams from the high energy transmission system; another method of control, which can be used in conjunction with the first, uses ambient radiation monitors to control the x-ray intensities from the x-ray generators.